The present invention is directed to a prospecting system and more specifically to the determination of subsurface resistivity to locate both mineral deposits and geothermal systems and in general to delinate zones of contrasting electrical resistivity that may be used to determine geological structures favorable to the accumulation of petroleum, ground water or natural gas.
Electromagnetic techniques have been used to determine subsurface resistivity which include the use of either an insulated wire grounded at each end and driven by an alternating current source or a multiturn loop of wire (magnetic dipole) also driven with alternating current to induce currents in the ground under the theory of Faraday's law. Such induced fields were detected either with a grounded wire, with a sensitive magnetometer or an induction coil.
The theory of the response of a conductive earth to simple dipole sources is relatively old. Quasi-static solutions have been made for one and two layer earths. In general, theoretical papers have presented complete electric and magnetic field solutions for vertical and horizontal magnetic dipoles and horizontal electric dipoles on the surface of N layered half spaces. The solutions are presented either in the frequency domain or in the time, or transient, domain.
Electromagnetic techniques have seen their greatest practical implementation in Western countries in airborne and ground exploration at relatively shallow depths (for example, less than 300 meters) for mineral exploration. These surveys are largely qualitative in the sense that distortions of the electromagnetic field cause by buried resistivity inhomogenities are detected. Such a system is illustrated in U.S. Pat. No. 3,836,841 in the name of the present inventor.
Deeper and more quantitative electromagnetic soundings, with correspondingly lower frequencies of operation (for example, 0.01 Hz to 100 Hz) have been used extensively for petroleum exploration in the Soviet Union.
Other methods have been used to measure the electrical resistivity to depths of several kilometers. DC sounding, for example, as used by the Schlumberger Corporation makes use of a system in which dc current is injected into the ground between two electrodes and the voltages produced are detected on a second distant pair of electrodes. The depth of exploration depends on the separation of these current and voltage electrodes. There is great difficulty in sounding to great depths; viz, the spacing required becomes so large that lateral changes in geology invariably invalidate the assumptions used in interpreting the data in terms of a horizontally layered model.
Another method, magnetotelluric (MT) makes use of the fact that the ratio of the natural low frequency electric, E, and magnetic, H, fields at the surface of the earth can be used to infer the resistivity as a function of depth in the earth. This method is also very sensitive to lateral resistivity variations and moreover accurate data is difficult to obtain due to the presence of local noise sources in either E or H which contaminate the natural fields.
The controlled source electromagnetic techniques avoid the problems associated with the above two techniques. However, there have been several practical problems in conducting successful exploration projects with them. Principally, there is the problem of generating strong enough fields at low frequency to penetrate to the depths required and to be measurable by sensitive detectors. The only commercial application in the U.S. (by Group Seven Inc. of Denver, Colo.) uses the grounded wire source. To permit the injection of sufficient current, up to 500 amperes, large electrode pits must be machine dug and up to two or three kilometers of heavy cable used with a 600 kilowatt generator. This source is obviously not convenient to emplace or move, it is often difficult to arrange permission for its emplacement, and it also presents a safety hazard in some cases. Finally, there are areas of high surface electrical resistivity where the electrode contact resistance cannot be reduced and sufficient source current cannot be injected.
In summary it has been realized that electro-magnetic techniques are useful in the exploration for petroleum, geothermal resources and deeply buried mineral deposits. Existing systems are difficult to use and, thus, an effective electromagnetic system is desirable which is of a relatively lightweight field transportable design and uses a loop source (magnetic dipole) so that no electrode contacting problems can arise.